"I've lost track of the many reasons that have been given for the switch to Intel, but this I know for sure: no one has ever reported that, for 18 months, Project Marklar existed only because a self-demoted engineer wanted his son Max to be able to live closer to Max's grandparents." Amazing.

Marklar wasn't really a secret. It was in plain sight: the OS, the build tools and the drivers were always out there in Darwin. The initial releases of what was to become Mac OS X, codename Rhapsody, were always available also as Intel builds and PPC actually came in later builds. As Mac OS X was released (even in beta), due to the popularity of Linux and OSS in the late 1999 dot-com boom, Apple made Darwin available. It was (and in my opinion it still is) a PR stunt.

The only things missing from Darwin were the OSX user land and some accelerated video drivers. Vesa FB drivers were always provided as an example driver. Mac OS X for Intel was ALWAYS a cross-compile away for some apps and 90% was always there, the 90% that was hardware dependent known as XNU.

In order to get from Darwin to OS X on Intel they had to write accelerated video drivers for the cards in the Macs and just recompile the non-BSD user land for x86. The kernel was continuously publicly available in x86 flavour with the BSD userland and the compiler (GCC) always produced x86 code. That's 99% of the porting work and it was public.

Relevant quotes from wikipedia:

Darwin does not include many of the defining elements of Mac OS X, such as the Carbon and Cocoa APIs or the Quartz Compositor and Aqua user interface, and thus cannot run Mac applications. It does, however, support a number of lesser known features of Mac OS X, such as mDNSResponder, which is the multicast DNS responder and a core component of the Bonjour networking technology, and launchd, an advanced service management framework.

Up to Darwin 8.0, Apple released a binary installer (as an ISO image) after each major Mac OS X release that allowed one to install Darwin on PowerPC and Intel x86 computers as a standalone operating system. Minor updates were released as packages that were installed separately. Darwin is now only available as source code,[4] except for the ARM variant, which has not been released in any form separately from iOS. However, the older versions of Darwin are still available in binary form.

So, the story should be summarised as: manager amazed that bored engineer was able to recompile a apps and libraries.